builtin/log: fix remaining -Wsign-compare warnings

builtin/log: fix remaining -Wsign-compare warnings

Fix remaining -Wsign-compare warnings in "builtin/log.c" and mark the
file as -Wsign-compare-clean. While most of the fixes are obvious, one
fix requires us to use `cast_size_t_to_int()`, which will cause us to
die in case the `size_t` cannot be represented as `int`. This should be
fine though, as the data would typically be set either via a config key
or via the command line, neither of which should ever exceed a couple of
kilobytes of data.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>